Navigation system using satellites and passive ranging techniques

ABSTRACT

A navigation system wherein the navigator&#39;&#39;s location is obtained by determining the navigator&#39;&#39;s distance (or range) from one or more satellites of known location. Each satellite transmits multifrequency signals that are derived from a stable oscillator which is phase synchronized with the navigator&#39;&#39;s equipment that produces similar multifrequency signals. Phase comparison between the signals received from the satellites and the locally produced signals indicates both the distance between the navigator and the satellites and the navigator&#39;&#39;s location. In determining his location, the presence of the navigator is not revealed since no interrogatory transmission by him is required.

United States Patent Easton [451 Jan. 29, 1974 [76] Inventor: Roger L.Easton, 7704 Oxon Hill Rd., Oxon Hill, Md. 20021 [22] Filed: Oct. 8,1970 [21] Appl. No.: 79,307

[52] US. (11.343/112 R, 343/100 ST, 343/112 D [51] lint. Cl. G01s 5/14,GOls 11/00 [58] Field of Search 343/112 D, 112 R, 100 ST [56] ReferencesCited UNITED STATES PATENTS 3,643,259 2/1972 Entner 343/112 D X3,384,891 5/1968 Anderson 343/100 ST UX 2,947,985 8/1960 Cooley 343/112D UX 3,397,400 8/1968 Maass et al. 343/112 D UX 2,924,820 2/1960 Dishalet al. 343/112 D UX 3,339,202 8/1967 Earp 343/112 D X SATELLITE PrimaryExaminer-Richard A. Farley Assistant Examiner-Richard E. BergerAttorney, Agent, or Firm-R. S. Sciascia; Arthur L. Branning; J. G.Murray [5 7] ABSTRACT A navigation system wherein the navigatorslocation is obtained by determining the navigators distance (or range)from one or more satellites of known location. Each satellite transmitsrnultifrequency signals that are derived from a stable oscillator whichis phase synchronized with the navigators equipment that producessimilar multifrequency signals. Phase comparison between the signalsreceived from the satellites and the locally produced signals indicatesboth the distance between the navigator and the satellites and thenavigators location. In determining his location, the presence of thenavigator is not revealed since no interrogatory transmission by him isrequired.

6 Claims, 3 Drawing Figures SATELLITE EARTH PATENTED 3.789.409

SHEET 1 BF 2 SATELLITE SATELLITE EARTH FIG.

STABLE 5M0 MULTIPLIER a 400 MC CARR'ER OSCILLATOR SYNTHESIZER l00m300m-- coum DOWN 3Kc-. MODULATOR gggmg CIRCUITS 12 8: l6 CARRIERFREQUENCY MAY VARY 0N DIFFERENT SATELLITES INVENTOR.

ROGER L. EASTON BY 92:42 7' TOR/VEYS PAIENTED 3.789.409

SUEU 2 of 2 STABLE OSCILLATOR |oo- |oo- 3oo- 3oo------ IKC IKC COUNTRECEIVER DOWN 3 KC 3K0 AND CIRCUITS KC IOKC FILTERS 3OKC 30 KC |oo KCI00 KC RECORDERS IO-NAVIGATORS STATION INVENTOR.

ROGER L. EASTO/V k i9 ATTORNEYS NAVIGATION SYSTEM USING SATELLITES ANDPASSIVE RANGING TECHNIQUES STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Until recently, celestial navigationtechniques have provided mankind with the best available accuracy forterrestrial navigation. A new order of accuracy has become possible withthe introduction of artificial earthsatellite navigation systems whereinthe instantaneous location of the satellites is precisely known.

Prior earth-satellite navigation systems have, in general, eitherincluded the measurement of range rate (doppler frequency shift) of cwtransmissions by the satellite or measured range by measuring the timeinterval between interrogation and response by a transponder on thesatellite. It has been found that range is the better parameter to use.Unfortunately, the mea surement of range has hitherto required thenavigator to radiate an interrogation signal (and thereby betray hispresence) and also raises the possibility that the satellite transponderwill saturate from a plurality of simultaneous interrogations bydifferent navigators.

SUMMARY OF THE INVENTION The present invention provides the advantagesof the prior earth-satellite navigation systems while avoiding theirdisadvantages. More specifically, the present invention allows thenavigator to passively determine his position by measuring the distance,or range, to one or more satellites. Each satellite transmitsmultifrequency signals which are derived from extremely preciseoscillators. Similar multifrequency signals are derived by thenavigators equipment from an extremely precise oscillator which is phasesynchronized with the oscillators on the satellites. By measuring thephase differences between the signals received from the satellites andthe locally produced reference signals, the navigator obtains anindication of the distance to the satellites and, therefrom, of his ownlocation. The navigators presence is not betrayed since no interrogationsignal transmission is required.

OBJECTS OF THE INVENTION It is, therefore, an object of the invention toprovide an improved earth-satellite navigation system.

A further object is to provide an imporved earthsatellite navigationsystem wherein range is measured without the necessity of transmittingan interrogatory signal.

Yet another object is the provision of an improved earth-satellitenavigation system wherein range is measured without the necessity oftransmitting an interrogatory signal by comparison of the phase ofsignals received from the satellites and locally generated referencesignals.

A still further object of the invention is to provide an improvedearth-satellite navigation system and technique wherein both thesatellites and the navigators equipment include precisely stableoscillators which are phase locked and wherein range is measured withoutthe necessity of transmitting an interrogatory signal by comparison ofthe phase of signals received from the satellites and locally generatedsignals.

DESCRIPTION OF THE DRAWINGS Other objects and advantages of theinvention will hereinafter become more fully apparent from the followingdescription and the annexed drawings, which illustrate a preferredembodiment of the invention, and wherein:

FIG. I is a perspective view of the invention;

FIG. 2 is a block diagram of the equipment on the satellite portion ofthe invention, and

FIG. 3 is a block diagram of the: equipment located at the navigatorsstation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before proceeding with adescription of the drawings wherein like reference characters designatelike or cor responding parts throughout the several Figs, the reader iscautioned that the drawings, and in particular FIG. 1, should not beconstrued as being more than merely representative of a practicalsystem. Obviously, much of the drawings and many of the features of thedisclosed embodiment are dimensionally distorted and extremelysimplified for the purpose that the disclosure of the invention might bepresented in a manner having clarity of illustration and description.

Referring now to FIG. 1, the navigator of concern, who is using thetechniques and equipment of the invention, is located on the ship 10.The navigator, knowing the precise coordinates of a first satellite 12(from tables prepared by procedures which are not, per se, a part ofthis invention) determines the range of the satellite by means andmethods yet to be described and, from this knowledge of satelliteposition and range, is able to determine a first line of position (LOP)14 for the ship 10. The same procedures are followed with respect to asecond satellite 16 to determine the second LOP 1B. The intersection ofthe LOPs l4 and 18, of course, define the location of the ship 10 withan accuracy of an order of magnitude greater than is obtainable byconventional navigation practices.

It is apparent that comparably good results (but not instantaneousnavigational fixes") can be obtained by using only one swiftly orbittingsatellite and determining two LOPs at different times.

The transmitting equipment on the satellites l2 and 116 is substantiallyidentical and is illustrated in block diagram form in FIG. 2. Forpurposes of signal identfication, the satellites l2 and 16 may transmiton different carrier frequencies. Further, as the reader no doubtrealizes, the use of specific frequencies in the following descriptionis merely for the purposes of description and, while the mentionedfrequencies may be typical, should not be considered as in any waylimiting the invention. Obviously, the invention could incorporate manycarrier and modulating frequencies different from those set forth in thefollowing description.

In FIG. 2 the stable oscillator 20 is controlled by an atomic clock (oris a crystal oscillator which has an equivalent frequency stability) andtypically produces a precise 5 MC signal. Preferrably, means areincluded (but not shown) which allow phase adjustments in this 5 MCsignal to correct for any phase drift that does occur.

The 5 MC output signal of oscillator 20 is connected to count downcircuits 22 which typically produce 100 cps, 300 cps, 1 KC, 3 KC, KC, 30KC and 100 KC signals. The output of oscillator is also connected tomultiplier and synthesizer circuits 24 which produces the 400 MC carriersignal that is modulated by the seven output signals of count downcircuits 22 in the modulator 26 and then transmitted by the satelliteantenna 28. It is worthy of emphasis that all of the modulating signalsare derived from the common source, i.e., oscillator 20.

Referring now to FIG. 3 which is a block diagram of the equipment at thenavigators station on the ship 10. The multifrequency modulated carriersignal from one of the satellites 12 or 16 is intercepted by antenna 30and, by techniques which are extremely well known, converted by receiverand filter system 32 into 100 cps, 300 cps, 1 KC, 3 KC, 10 KC, 30 KC and100 KC signals. Care must of course be taken, according to the bestknown design practices, to avoid the introduction of even small phaseshifts into any of the seven output signals of the receiver and filtersystem 32.

Stable oscillator 34 is similar in frequency stability to the oscillator20 (FIG. 2) and typically produces a 5 MC signal which is keptsynchronized or in phase coincidence with the signal produced byoscillator 20. The reader will appreciate that oscillator 34, being onship 10 where weight is relatively not critical, would typically becontrolled by an atomic clock whereas oscillator 20, being on asatellite where weight is extremely critical, might be an ultra stablecrystal oscillator. Oscillator 34 is connected to count down circuits 36which typically produce 100 cps, 300 cps, 1 KC, 3 KC, 10 KC, 30 KC and100 KC output signals. The count down circuits 36 are functionallysimilar to the count down circuits 22 (FIG. 2) but may be very differentin size and weight because the circuits on the satellite will, forobvious reasons, be designed to be small and light.

Phase meters 38 are connected to receive the seven output signals fromreceiver and filtering system 32 and the seven reference signals fromcount down circuits 36 and function to energize recorders 40 to providea record of the phase difference between (say) the 3 KC signals fromboth system 32 and circuits 36, etc. The seven individual phasecomparisons at differing frequencies provided by recorders 40 enable thenavigator to determine the range of the satellite without ambiguity andwith good resolution. In other words, the phase comparison of the lowfrequencies are not ambiguous but do not provide high accuracy while thephase measurement of the high frequency signals, which are ambiguousbecause the record will not distinguish phase differences of 20, 380,740, etc., will provide measurements of high accuracy.

The operation of the disclosed preferred embodiment of the invention isby now apparent. Highly stable oscillators 20 and 34 (which arerespectively on the satellites 12 and 16 and on the navigators ship 10)produce 5 MC signals which are practically phase locked and which arechanged by count down circuits 22 and 36 (which are respectively on thesatellites 12 and 16 and on the navigators ship 10) to l00-cps, 300 cps,1 KC, 3 KC, 10 KC, 30 KC and 100 KC signals which are also, frequency byfrequency, practically phase locked. 6

from the 5 MC oscillator signal. Each of the modulated carrier signalsis broadcast and is selectively received (the carrier frequency on eachsatellite being different) by the equipment at the navigators station onship 10.

After demodulation by the receiver and filter system 32, the sevencomponent signals are compared, frequency by frequency, with the sevenreference signals produced by count down circuits 36, the phasedifferences of which are indications of the transit time of the signalstransmitted by the satellites and hence a measure of the range of thesatellites. Since the navigator knows (from prepared tables) the exactlocation of the satellites l2 and 16, it is possible to determine fromthe ranges of the satellites the LOPs 14 and 18 and to determine theposition of the navigators ship 10 from the intersection of the LOPs 14and 18.

There has been disclosed an improved earth satellite navigation systemwherein both the satellites and navigators equipment include preciselystable oscillators which are phase locked and wherein range is passivelymeasured, without the necessity of transmitting an interrogatory signal,by comparison of the phase of signals received from the satellites andlocally generated signals. Obviously many modifications and variationsof the present invention are possible in the light of the aboveteachings. For example, the navigators station could obviously be on aflying airplane rather than on surface ship 10 as described. It istherefore to be understood, that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed.

What is claimed and desired to be secured by letters patent of theUnited States is:

l. A navigation system comprising:

at least two earth satellites, the instantaneous locations of which areprecisely known, each satellite including transmitting means forbroadcasting multifrequency signals derived from a first extremelystable oscillator, and

a navigators station including:

receiving means for receiving said broadcast multifrequency signals;

signal generating means including a second extremely stable oscillatorfor producing multifrequency signals the same as said broadcast signals,

and

phase comparison means connected to said receiving means and to saidsignal generating means for comparing the phase of said receivedmultifrequency signals to the phase of said produced multifrequencysignals whereby the location of said navigators station can bedetermined from the intersection of lines of position established bydetecting the stations distance from the instantaneously known locationof the satellites derived from said phase comparisons without thenecessity of betraying the presence of said navigators station by aninterrogation signal.

2. The technique of determining a navigators location by determining hisposition relative to a moving satellite whose location is known at alltimes comprising the steps of:

measuring the phase difference between multifrequency signals receivedfrom said satellite at first and second locations and reference signalsof corresponding frequencies produced at said navigators location, saidsignals including a low frequency signal and a signal having a frequencyat least one hundred times higher than the low frequency signal;

determining approximate first and second distances between saidnavigator and said satellite at the first and second locations,respectively, from the phase comparison of said low frequency signals;

determining accurate first and second distances from said approximatedistances and the phase comparison of the higher frequency signals;

establishing first and second lines of position of said navigator fromsaid accurately determined distances,

whereby said navigators location is determined by the intersection ofsaid first and second lines of position.

tor and which produce said carrier frequency signal.

1. A navigation system comprising: at least two earth satellites, theinstantaneous locations of which are precisely known, each satelliteincluding transmitting means for broadcasting multifrequency signalsderived from a first extremely stable oscillator, and a navigator''sstation including: receiving means for receiving said broadcastmultifrequency signals; signal generating means including a secondextremely stable oscillator for producing multifrequency signals thesame as said broadcast signals, and phase comparison means connected tosaid receiving means and to said signal generating means for comparingthe phase of said received multifrequency signals to the phase of saidproduced multifrequency signals whereby the location of saidnavigator''s station can be determined from the intersection of lines ofposition established by detecting the station''s distance from theinstantaneously known location of the satellites derived from said phasecomparisons without the necessity of betraying the presence of saidnavigator''s station by an interrogation signal.
 2. The technique ofdetermining a navigator''s location by determining his position relativeto a moving satellite whose location is known at all times comprisingthe steps of: measuring the phase difference between multifrequencysignals received from said satellite at first and second locations andreference signals of corresponding frequencies produced at saidnavigator''s location, said signals including a low frequency signal anda signal having a frequency at least one hundred times higher than thelow frequency signal; determining approximate first and second distancesbetween said navigator and said satellite at the first and secondlocations, respectively, from the phase comparison of said low frequencysignals; determining accurate first and second distances from saidapproximate distances and the phase comparison of the higher frequencysignals; establishing first and second lines of position of saidnavigator from said accurately determined distances, whereby saidnavigator''s location is determined by the intersection of said firstand second lines of position.
 3. The navigation system of claim 1wherein no more than one satellite broadcasts at any one carrierfrequency.
 4. The navigation system of claim 3 wherein said receivingmeans functions to separate said multifrequency signals from the carrierfrequency which is broadcast.
 5. The navigation system of claim 4wherein said signal generating means includes count down circuits whichare connected to said second stable oscillator.
 6. The navigation systemof claim 5 wherein said transmitting Means includes multiplier andsynthesizer means which are connected to said first stable oscillatorand which produce said carrier frequency signal.